1. Field of the Invention
This invention relates to temperature-actuated alarm devices, and is directed more particularly to small, portable, self-contained, gas pressure-powered, temperature-sensitive alarm systems.
2. Description of the Prior Art
Temperature-sensing of fire alarm systems incorporating gas pressure-powered sound-emitting devices, and activated by a temperature-sensing element, have found widespread use in commercial and residential applications. In order for these systems to provide the intended protection, it is necessary that they reliably produce a loud warning signal and, further, that the warning signal be of sufficient duration that persons intended to be alerted by the signal have ample time to take notice of it. Certain consumer testing organizations that provide certification for such devices and labeling attesting to the fact that the devices meet their quality standards, require that certification be conditioned upon a duration of signal of at least 4 minutes (at a sound pressure level of 85 decibels or greater).
A number of commercially-available alarm devices using a gas pressure-powered horn meet this duration of signal requirement by providing a large pressure vessel and quantity, i.e., a pound or more, of liquified gas propellant. The large pressure vessel has disadvantages, however, in its size and in that it generally contains much more propellant than is actually necessary to drive a horn of a suitably loud type for the required time. As a result, the alarm device is more costly to manufacture, due to the larger pressure vessel and extra propellant, and it is also bulky. The theoretical propellant requirements of a small, but sufficiently loud horn, in terms of the total amount of gas required to sound the horn for the minimum time, may be met with a much smaller reservoir. We have found that in accordance with our invention, an adequate amount of liquified propellant may be contained in a reservoir of about 6 ounces nominal capacity to meet the required quality standards.
When an alarm device of the type above described and using a propellant reservoir of about 6-ounce size is triggered at a temperature of about 136.degree.F, the horn sounds for only a few seconds and then stops. Since the initial rapid vaporization of propellant withdraws heat rapidly from the remaining liquid propellant, according to well-known principles, it is likely that this unsatisfactory performance is due to the lack of sufficiently rapid heat transfer from the surroundings into the small propellant reservoir to maintain vaporization of the remaining cold propellant at a rate sufficient to provide adequate gas flow to the horn.